Reciprocating piston internal-combustion engines that are in wide use today are predominately of the four-stroke cycle type, although the two-stroke cycle type with crankcase charging is also quite common.
Four-stroke cycle internal-combustion engines were patented by Alphonse Beau de Rochas in France in 1862. He specified using a single cylinder and set forth the following mode of operation:
I. Drawing in the charge during one whole piston stroke. PA1 II. Compression during the following stroke. PA1 III. Inflamation at the dead point, and expansion during the third stroke. PA1 IV. Discharge of the burnt gases from the cylinder during the fourth and last stroke.
Engines employing the four-stroke operating cycle were first manufactured by Nicolaus Otto in Deutz, Germany in 1876 on which he was granted a patent in 1877. They operate on a mixture of air and a fuel gas or a volatile liquid fuel such as gasoline, kerosene, or alcohol and have been very successful.
The paradox is that although the engines derive their power by the expansion of heated gases from the combustion of a fuel-air mixture within a combustion chamber they are intolerant of the residual heat that is absorbed by the surfaces of the combustion chamber. Instead of using the residual heat to increase the pressure of the next fuel-air charge, the chamber must be cooled sufficiently to prevent preignition; that is, ignition of the charge from a hot surface before the piston is at the proper place approaching the top of its travel. Further, the chamber must be cooled to aid in the prevention of detonation that is caused by autoignition of the end gas, which is the yet unburned part of a burning charge.
Finally, even if preignition and detonation had not made cooling necessary, the engine is not arranged to extract power from the residual heat because the increased work of compression in a hot cylinder cancels the increased work output.
The charge must be compressed (or confined) before heat is added to it if there is to be any net work realized from the heating.
Volumetric efficiency of induction suffers a loss of about 5% in a liquid cooled engine from heating of the charge during the intake stroke with greater losses from hotter operation in air cooled engines.
Two-stroke cycle engines of the Day model, in which a charge of fuel, lubricating oil, and air is inducted into the crankcase and transferred into the motor cylinder while the piston is down for subsequent compression, are also susceptible to all of the foregoing problems.
Most diesel engines have a four-stroke cycle with late cylinder injection in which a charge of air is compressed into one-fourteenth or less of its atmospheric volume so as to heat it sufficiently to initiate combustion when fuel is injected into it at the proper time. They are, accordingly, not susceptible to preignition and detonation in the manner of carbureted engines; however, they are susceptible to similar losses in volumetric efficiency and increased work of compression in a hot cylinder. These losses undermine attempts to increase the efficiency of diesel engines by "low heat rejection" (hotter) operation, which has also been called adiabatic operation; however, adiabatic refers to an operation without loss or gain of heat from the enclosure; this is not the case when air is inducted into and compressed in a hot cylinder where heat transfer is intense and the work of compression is substantially increased.
In addition to being unable to use all of the heat generated by the combustion, the bulk of a power plant is substantially increased by a cooling system, and engines have been forced to expend part of the power generated to remove a substantial portion of the absorbed heat and to transport unwieldy cooling systems.
Detonation has came to be of paramount importance in both four-cycle and two-cycle carbureted engines; that is, engines in which fuel is added to the air by any means before the inlet port to the compressing cylinder is closed. In addition to the requirement that the combustion chamber be kept as cool as practical, which leads to the adoption of liquid cooling systems, it limits the compression ratio to a level that will not detonate with the available fuel and thereby necessitates the production of expensive fuels with high octane ratings, which are a measure of their resistance to detonation. It also leads to the use of antidetonation (antiknock) agents such as tetraethyl lead and requires the use of extremely rich fuel-air mixtures when maximum resistance to detonation is necessary such as an aircraft at take off power as well as compromising the shape of the combustion chamber and promoting the use of water or alcohol injection.
There have been engines made that compress the charge with a separate pump cylinder and then burn and expand it against a motor piston in a motor cylinder. Their general mode of operation can be called a two-stroke pump-compression cycle, but none have had lasting success. Several kinds have been tried.
One was patented in 1872 by George Brayton of Boston, Mass. Another, the Simon, was an English adaptation of it. The Brayton used a pump to compress a mixture of fuel and air into a receiver tank from which it was valved through a wire gauze flame barrier into a motor cylinder in which it was ignited upon entry for cursive combustion; that is, the charge burned as it flowed into the combustion chamber. The volume of the charge increased from the heating, but the pressure could not increase from that in the receiver (constant-pressure combustion). It was commercially successful for a period, but later it could not compete with the Otto engine, which was much more efficient because it operated at higher pressures achieved with constant-volume combustion.
Another engine having a pump cylinder and a motor cylinder was the Wittig & Hees which was made in Germany around 1880. Its pistons were connected to crankpins set at the same angle. When the pistons were 40% of the stroke from full compression, the exhaust valve closed and the fresh charge was pushed into the motor cylinder to be compressed with the hot residual gas. Ignition was at the dead point (top dead center). If the engine was working hard with the fuel-air mixture lean, preignition from the compressed residual gas seems likely, but perhaps it operated satisfactorily with the very low compression ratios used in, that period with a normally rich fuel-air mixture. Because its ignition was delayed until the entire charge was in the motor cylinder and fully compressed, it would have been subject to detonation had the compression ratio been higher, so it offered no advantage over the Otto type. This engine had a two-piston cycle having constant-volume combustion; however, charging started far too early for cursive combustion to be used.
Foulis of Glascow, Scotland brought out an improved version of his engine having a motor cylinder and pump in 1881. Its charge was compressed in the pump and valved through a wire gauze flame barrier and a regenerator into a red hot combustion chamber with cursive combustion. Admission continued until one third of the motor stroke was completed for constant-pressure combustion. The volume of the regenerator and passages further prevented the high pressure constant-volume combustion necessary for efficiency.
As discussed in the last few paragraphs, engines of diverse characteristics have been included in the two-stroke pump-compression cycle classification. The distinguishing characteristic linking them will herein be called "upper-cylinder pressure charging"; defined as the forcible transfer under pressure by a pump of fresh working fluid into the main cylinder after the exhaust port is closed and while the motor piston is in the upper half of its travel, which thereby raises the pressure materially above atmospheric pressure.
The expression "pump-compression cycle engine" will hereinafter mean only that the engine being described has a pump piston operating in a cylinder and a motor piston operating in a cylinder with upper-cylinder pressure charging.
A persistant school of thought seems to have arose during the nineteenth century wherein it was desired to use the heat of combustion only to increase volume rather than to increase pressure, probably because stronger engines would be necessary for the higher pressures. Isothermal combustion was favored by Rudolph Diesel in 1897 when developing his compression-ignition engine wherein the maximum temperature attained during compression of the air was not intended to be exceeded by the temperature during combustion. This approach allows the pressure to decrease as volume increases even while the fuel is being burned and gives even lower efficiency than with constant-pressure combustion.
The major contribution to efficiency of constant-volume combustion, wherein the temperature and pressure attained during compression are greatly increased by the combustion of fuel with an accompanying increase in work output and efficiency that is obtained from the higher pressure upon the piston as it moves downward, was not yet obvious, although Beau De Rochas had specified it; and Otto had already put it into practice.
The higher efficiency that can be derived from constant-volume combustion may be easily seen from the fact that the temperature increase in a compressed charge of air will be 1.4 times as great by adding a fixed quantity of heat at constant volume rather than at constant pressure from the ratio of the heat capacity at constant pressure to that at constant volume, which is 1.4 for air; and then using the expression derived from the work of Sadi Carnot in 1824 for the maximum possible efficiency of any engine working between two temperatures: EQU e=1-T.sub.2 /T.sub.1
which relates the maximum temperature during the cycle of the compressed charge after the addition of heat, T.sub.1, to the minimum temperature after expansion when the exhaust valve is about to open, T.sub.2, assuming that the expansion will be carried out to the same temperature in both cases, which will require greater expansion for the constant pressure engine while still giving poorer efficiency.
The demise of double piston engines having compression in one cylinder and combustion in another was perhaps partly because they were not restricted to constant-volume combustion as was the Otto type, and apparently, designers of the period would opt for constant-pressure combustion when allowed a choice. More difficult to solve are the inherent problems of valving high-pressure gases, effecting total transfer of a fuel-air charge to the motor cylinder and preventing combustion in the charger cylinder. There have been many patents issued over the years for engines working on the two-stroke pump-compression cycle, but none have solved the inherent requirements as well as Otto solved the inherent requirements of the Beau de Rochas cycle.
After Otto's patent expired, the other manufacturers adopted engine designs having all high pressure operations in one cylinder, either the four-stroke cycle type such as his or the two-stroke cycle type such as the Day model with crankcase charging, both of which compress the charge in the hot motor cylinder, which leaves them susceptible to preignition and detonation, the bane of high compression carbureted engines, and necessitates the extraction and waste of the heat absorbed by the internal surfaces of the engine.
Meanwhile, the gas turbine engine has been designed so that it achieves excellent efficiency, even with constant-pressure combustion, by using incoming air to cool itself while increasing the volume of the air without wasting heat into a cooling system. Its reciprocating-piston-engine analogue, the internal-combustion pump-compression cycle engine, should be capable of even greater efficiency, particularly when it is permitted the additional advantage of high-pressure-constant-volume combustion of a premixed fuel-air charge.
There is no theoretical requirement for the waste of heat by an internal-combustion piston engine other than that in the exhaust gas, and that may be minimized by using a large compression ratio combined with a larger expansion ratio, which may be done at will when designing an internal-combustion pump-compression cycle engine.